New observations on the development of the gonadotropin-releasing hormone system in the mouse

In ongoing efforts to study the ontogeny of gonadotropin-releasing hormone (GnRH) neurons, we serendipitously observed that increasing times of incubation in antibodies enhanced signal detection. Here, we describe significant differences in the early migration pattern, population dynamics, and growth cone morphology from published reports. The first immunoreactive GnRH cells were detected in the mouse at E10.75 (7.6 ± 2.8 cells; morning after mating = E0.5), prior to the closure of the olfactory placode. Although half of these cells were in the medial wall of the olfactory pit, the other half had already initiated their migration, and approximately one quarter had reached the telencephalic vesicle. Although the migratory pattern of the GnRH cells after E11.00 was identical to that described previously, these earliest migrating cells traveled singly rather than in cords, with some reaching the presumptive preoptic area (posterior to the ganglionic eminence) by E11.75. The number of GnRH cells increased significantly (p < 0.05) to 777 ± 183 at E11.75 and peaked at 1949.6 ± 161.6 (p < 0.05) at E12.75. The adult population was approximately 800 cells distributed between the central nervous system (CNS) and the nasal region. Hence, the population of GnRH neurons during early development is much larger than previously appreciated; mechanisms for its decline are discussed. Neuritic extensions on the earliest GnRH neurons are short (30–50 μm) and blunt and may represent the leading edge of the moving cell. By E12.75, GnRH axons in the CNS had a ribboned or beaded morphology and increasingly more complex growth cones were noted from this time until the day of birth. The most complex growth cones were associated with apparent choice points along the axons' trajectory. By E13.75, GnRH axons were seen at the presumptive median eminence in all animals, and it was at this stage that the axons began to branch profusely. Branching, as well as the presence of growth cones, continued postnatally. These results provide further insights into the pathfinding mechanisms of GnRH cells and axons. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 983–998, 1997     

The role of Paraxial Protocadherin in Xenopus otic placode development

Vertebrate inner ear develops from its rudiment, otic placode, which later forms otic vesicle and gives rise to tissues comprising the entire inner ear. Although several signaling molecules have been identified as candidates responsible for inner ear specification and patterning, many details remain elusive. Here, we report that Paraxial Protocadherin (PAPC) is required for otic vesicle formation in Xenopus embryos. PAPC is expressed strictly in presumptive otic placode and later in otic vesicle during inner ear morphogenesis. Knockdown of PAPC by dominant-negative PAPC results in the failure of otic vesicle formation and the loss of early inner ear markers Sox9 and Tbx2, suggesting the requirement of PAPC in the early stage of otic vesicle development. However, PAPC alone is not sufficient to induce otic placode formation.     

Hindbrain-derived Wnt and Fgf signals cooperate to specify the otic placode in Xenopus

Induction of the otic placode, the rudiment of the inner ear, is believed to depend on signals derived from surrounding tissues, the head mesoderm and the prospective hindbrain. Here we report the first attempt to define the specific contribution of the neuroectoderm to this inductive process in Xenopus. To this end we tested the ability of segments of the neural plate (NP), isolated from different axial levels, to induce the otic marker Pax8 when recombined with blastula stage animal caps. We found that one single domain of the NP, corresponding to the prospective anterior hindbrain, had Pax8-inducing activity in this assay. Surprisingly, more than half of these recombinants formed otic vesicle-like structures. Lineage tracing experiments indicate that these vesicle-like structures are entirely derived from the animal cap and express several pan-otic markers. Pax8 activation in these recombinants requires active Fgf and canonical Wnt signaling, as interference with either pathway blocks Pax8 induction. Furthermore, we demonstrate that Fgf and canonical Wnt signaling cooperate to activate Pax8 expression in isolated animal caps. We propose that in the absence of mesoderm cues the combined activity of hindbrain-derived Wnt and Fgf signals specifies the otic placode in Xenopus, and promotes its morphogenesis into an otocyst.